Oil and gas pipelines located at Alaska's Prudhoe Bay are wrapped with a jacket of insulating material to prevent the rapid cooling, and provide better transportability, of oil and gas fluids. The outer surface of the insulation is covered by a metal jacket for keeping out moisture. The metal jacket is typically provided in two half portions with each portion having flanges for aiding in the retention of the jacket on the pipeline. The two half portions of the jacket are joined together at the flanges which form seams. Water occasionally enters through the jacket seams and travels through the insulation to the pipe where it causes corrosion.
Prior art methods of detecting pipeline corrosion have proven inadequate. For example, pigs with corrosion detection equipment can only be used on pipelines that have access locations; many pipelines lack such locations. Ultrasonic detection methods require removal of the metal jacket and insulation, a timely and expensive procedure. Radiography detection methods are potentially hazardous and the equipment is cumbersome, requiring impractical or inconvenient adjacent vehicular support. Furthermore, with radiography methods it is often difficult to distinguish between corrosion pits filled with corrosion products and uncorroded portions of pipe walls. What is needed then is a method of detecting corrosion through insulation and the surrounding jacket, and which method can be practiced with portable equipment.
Electromagnetic probing techniques provide such a method for detecting corrosion through insulation. In the prior art, frequency domain electromagnetic probing techniques are used to detect corrosion in aircraft fuel tanks. Frequency domain electromagnetic probing techniques utilize a small number of frequencies and measure magnitude and phase differentials between the transmitted signals and the received signals. However, because frequency domain techniques, as a practical matter, utilize only a small number of frequencies, the amount of information obtained is inherently limited, thus detracting from the accuracy of the techniques.
The application "Method for Detecting Corrosion On Conductive Containers" by Brian Spies, which application is assigned to the assignee of the present invention and which application is filed the same day as the present application, discloses time domain electromagnetic probing methods for use in detecting corrosion in conductive containers. The present application discloses a method of detecting corrosion in conductive containers wherein the distance between the antenna means and the container wall varies across the area of the conductor wall, typically due to variations in insulation thickness.
It is an object of the present invention to provide a method for detecting corrosion on insulated conductive containers, wherein said method has improved accuracy in detection and can detect corrosion through a jacket having variations in its thickness.